Absolute distance measurement apparatus and method using laser interferometric wavelength leverage

ABSTRACT

An absolute distance measurement apparatus and method using laser interferometric wavelength leverage includes a light source system, a wavelength-leverage laser interferometric system and an interference signal processing and controlling system. The light source system outputs a orthogonally linearly polarized beam with the wavelength λ 1  and the wavelength λ 2 . The orthogonally linearly polarized beam projects onto the wavelength-leverage laser interferometric system to form the interference beam. The interference beam projects onto the interference signal processing and controlling system. In the wavelength-leverage laser interferometric system, the synthetic wavelength and the single wavelength as well as the measured absolute distance and the moving displacement of the cube-corner prism in the reference arm form a wavelength-leverage absolute distance measurement relationship.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to an optical measuring apparatus andmethod and particularly relates to an absolute distance measurementapparatus and method using laser interferometric wavelength leverage.

Description of Related Art

With the development of science and technology, absolute distancemeasurement of large length with high accuracy are applied more and morewidely to high-end equipment manufacturing, spatial engineering,metrological technique and so on. For example, the measurement of racksof heavy machines, the measurement of beds of large precision machinetools, the measurement of lengths of steam and hydraulic turbinespindles, the measurement of diameters of stators and rotors ofwater-turbine generator sets of hydropower stations, the measurement ofinstalling positions of aircraft fixtures in the aerospace industry, themonitoring of positions and postures and the high-resolution distancemeasurement of satellites in satellite formation flying and so on notonly demand distance measurement accuracy to reach micrometer level andeven lower in the scope from few tens of meters to hundreds of metersbut also need these measurement instruments to have high efficiency andgood flexibility and to be suitable for measuring conditions withoutguide rails.

The absolute distance measurement methods are mainly divided into twotypes: time-of-flight measurement and interferometric measurement. Amongthe time-of-flight measurement, the distance measurement with laserpulses is limited by picosecond time measurement accuracy, and thus thedistance measurement accuracy is in the millimeter level; although themeasurement accuracy of the balanced optical cross-correlation based onfemtosecond pulses superposition can realize the sub-micrometer order,the measured distance must be an integer multiple of the femtosecondinterpulse distance, then arbitrary absolute distance cannot bemeasured; the phase shifting method achieves the time of flight bymeasuring the phase delay generated by the measured round-trippingdistance of modulated light waves, the measurement resolution depends onthe phase resolution of the maximum modulating frequency, and themeasured distance is limited by the non-ambiguity distance correspondingto the minimum modulating frequency. The laser interferometric absolutedistance measurement mainly includes frequency-sweeping interferometry,femtosecond pulse dispersive interferometry and multiple-wavelengthinterferometry. For frequency-sweeping interferometry, the variations ofthe measured distance will induce large errors during the sweep, and therelative uncertainty is 10⁻⁶; the femtosecond pulse dispersiveinterferometry is limited by the resolution of spectrum analyzer, andthe relative uncertainty is 10⁻⁵; the multiple-wavelength interferometryuses multiple wavelengths to constitute a gradually increasingsynthetic-wavelength chain and gradually obtains the measured distanceby starting from the maximum synthetic wavelength according to theinitial estimate of the measured distance and the fractional fringeorder corresponding to each synthetic wavelength. In themultiple-wavelength interferometry, the fractional fringe order isusually determined by using beat-wave detection, heterodyne detection,superheterodyne detection and son on, wherein the beat-wave detection isaffected by direct-current drift of light intensity, and relativeuncertainty is less than 10⁻⁶; in the optical configurations of theheterodyne detection and the superheterodyne detection, heterodyne lightsources are affected by frequency modulators and the stability ofsynthetic wavelength is low, and thus the distance measurement accuracyis difficult to improve.

BRIEF SUMMARY OF THE INVENTION

The present invention aims to disclose an absolute distance measurementapparatus and method using laser interferometric wavelength leverage.According to the one-to-one correspondence between the syntheticwavelength formed by two single wavelengths and a single wavelength, aleverage relation is formed between the measured absolute distance andthe moving displacement of the cube-corner prism in the reference arm.The measured absolute distance of large length is obtained bydetermining the moving displacement of the cube-corner prism in thereference arm. The present invention has the advantages of largemeasurement range, high measurement accuracy and traceability to thedefinition of the meter.

The present invention adopts the following technical solutions to solvethe technical problem:

1. An absolute distance measurement apparatus using laserinterferometric wavelength leverage:

comprising a light source system, a wavelength-leverage laserinterferometric system and an interference signal processing andcontrolling system. The light source system outputs a orthogonallylinearly polarized beam with the wavelength λ₁ and wavelength λ₂. Theorthogonally linearly polarized beam projects onto thewavelength-leverage laser interferometric system to form theinterference beam. The interference beam projects onto the interferencesignal processing and controlling system. The controller in theinterference signal processing and controlling system is used forcontrolling the change of the wavelength λ₂ in the light source system.

The light source system comprises a first laser, a second laser, a firstbeam expander, a second beam expander, a first reflector and a firstpolarizing beam splitter. The linearly polarized beam with a constantwavelength λ₁ emitted from the first laser passes through the first beamexpander and projects onto the first polarizing beam splitter. Thelinearly polarized beam with a variable wavelength λ₂ emitted from thesecond laser, whose polarization direction is perpendicular to that ofthe beam from the first laser, passes through the second beam expander,is reflected by the first reflector and projects onto the firstpolarizing beam splitter. The linearly polarized beam with the constantwavelength λ₁ transmitted by the first polarizing beam splitter and thelinearly polarized beam with the variable wavelength λ₂ reflected by thefirst polarizing beam splitter combine one orthogonally linearlypolarized beam.

The wavelength-leverage laser interferometric system comprises a firstbeam splitter, a second polarizing beam splitter, a first cube-cornerprism, a second beam splitter, a first shutter, a second shutter, asecond reflector, a third reflector, a third beam splitter, a secondcube-corner prism and a third cube-corner prism. Being incident on thefirst beam splitter, the orthogonally linearly polarized beam is dividedinto the reflected reference beam and the transmitted measurement beam.The reflected reference beam projects onto the second polarizing beamsplitter, wherein the linearly polarized beam with the wavelength λ₂ isreflected twice by the second polarizing beam splitter and projects ontothe first beam splitter, and the linearly polarized beam with thewavelength λ₁ passes through the second polarizing beam splitter,projects onto the first cube-corner prism, returns after beingreflected, and projects onto the first beam splitter after passingthrough the second polarizing beam splitter again. Being incident on thesecond beam splitter, the transmitted measurement beam is divided intothe reflected near-end measurement beam and the transmitted far-endmeasurement beam. The near-end measurement beam projects onto the firstbeam splitter after passing through the first shutter, the secondreflector, the second cube-corner prism and the third beam splitter insequence. And the far-end measurement beam projects onto the first beamsplitter after passing through the second shutter, the third cube-cornerprism, the third reflector and the third beam splitter in sequence.Being reflected by the first beam splitter, the near-end or far-endmeasurement beam recombines with the reference beam transmitted by thefirst beam splitter to form the interference beam.

The interference signal processing and controlling system comprises athird polarizing beam splitter, a first photodetector, a secondphotodetector, a data acquisition module, a computer and a controller.The interference beam from the wavelength-leverage laser interferometricsystem projects onto the third polarizing beam splitter. Theinterference beam with the wavelength λ₂ reflected by the thirdpolarizing beam splitter is received by the first photodetector, and theinterference beam with the wavelength λ₁ transmitted by the thirdpolarizing beam splitter is received by the second photodetector. Theinterference signals output by the two photodetectors are respectivelysent to the data acquisition module, and then transmitted to thecomputer after being processed by the data acquisition module. Accordingto the calculated result, the computer changes the value of thewavelength λ₂ emitted from the second laser by means of the controller.

In the light source system, the wavelength λ₁ of the laser beam emittedfrom the first laser is a constant value and the wavelength λ₂ of thelaser beam emitted from the second laser is a variable value.

In the wavelength-leverage laser interferometric system, the referencearm is composed of the second polarizing beam splitter and the firstcube-corner prism while the measurement arm is composed of the secondbeam splitter, the first shutter, the second shutter, the secondreflector, the third reflector, the third beam splitter, the secondcube-corner prism and the third cube-corner prism.

In the wavelength-leverage laser interferometric system, there is aone-to-one correspondence between the synthetic wavelength λ_(S) formedby the linearly polarized beams with the wavelengths λ₁, and λ₂ in themeasurement arm and the wavelength λ₁ of the linearly polarized beamprojecting onto the first cube-corner prism in the reference arm. Themeasured absolute distance in the measurement arm and the movingdisplacement of the first cube-corner prism in the reference arm form aleverage relationship.

2. An absolute distance measurement method using laser interferometricwavelength leverage: the method adopts the apparatus to performmeasurement, and the specific steps are as follows:

1) The first laser emits the linearly polarized beam with the wavelengthλ₁. Controlling the second laser to emit the linearly polarized beamwith the wavelength λ₂ makes one half of the primary syntheticwavelength λ_(S1) formed by the wavelengths λ₁ and λ₂ be larger than themeasured absolute distance L which is the distance between the secondcube-corner prism and the third cube-corner prism;

2) Adjust the open and close states of the first shutter and the secondshutter. Move the first cube-corner prism along the axial direction oflight path. Establish the wavelength-leverage relationship. The primarycoarse measurement value of the measured absolute distance is achieved;

3) Keep the wavelength λ₁ output from the first laser constant. Thecomputer changes the wavelength λ₂ output from the second laser by meansof the controller so that the wavelength λ₁ and the wavelength λ₂ form aseries of synthetic wavelengths λ_(S2), λ_(S3), . . . , λ_(Si), . . . ,λ_(Sn). The measurement is performed repeatedly with respect to eachchange of the wavelength λ₂. The fractional fringe order ε_(Si) and theintegral fringe order M_(Si) of the synthetic wavelength included in themeasured absolute distance L are obtained through calculation;

4) According to the synthetic wavelength and its fractional fringe orderε_(Si) and integral fringe order M_(Si) obtained in the above steps, thecomputer calculates each estimate value of the measured absolutedistance L during each measurement. The measured absolute distance L isobtained through calculation. Then the measurement of absolute distanceusing laser interferometric wavelength leverage is realized.

Step 2) specifically includes;

2.1) When the first shutter is opened and the second shutter is closed,the near-end measurement beam returned from the second cube-corner prismand the reference beam form the interference signals. Moving the firstcube-corner prism along the axial direction of light path makes thephase difference Δφ between the interference signals of wavelengths λ₂and λ₁ detected respectively by the first photodetector and the secondphotodetector be equal to zero;

2.2) When the first shutter is closed and the second shutter is opened,the far-end measurement beam returned from the third cube-corner prismand the reference beam form the interference signals. The phasedifference Δφ between the two interference signals received by the firstphotodetector and the second photodetector is changed. Moving again thefirst cube-corner prism along the axial direction of light path makesthe phase difference Δφ be equal to zero. The moving displacement of thefirst cube-corner prism is recorded as Δl;

2.3) The wavelength-leverage relationship among the primary syntheticwavelength λ_(S1), the wavelength λ₁ output from the first laser, themeasured absolute distance L and the moving displacement Δl of the firstcube-corner prism is established as follows:

$\frac{L}{\lambda_{S\; 1}} = \frac{\Delta \; l}{\lambda_{1}}$

where λ_(S1)=λ₁λ₂/|λ₁−λ₂| is the primary synthetic wavelength formed bythe wavelengths λ₁ and λ₂. λ₁ and λ₂ are the wavelengths output from thefirst laser and the second laser, respectively;

According to the wavelength-leverage relationship, the primary coarsemeasurement value L₁ of the measured absolute distance is calculated bythe computer using the following equation:

$L_{1} = {{\frac{\lambda_{S\; 1}}{\lambda_{1}} \cdot \Delta}\; {l.}}$

Step 3) specifically includes:

A series of synthetic wavelengths meets λ_(S2), >λ_(S3)> . . . >λ_(Sn),and each synthetic wavelength λ_(Si) meets

λ_(Si)>4u(L _(i-1)′), i=2,3, . . . ,n

where i represents the sequence number of measurement times, nrepresents the total number of measurement times, L_(i-1)′) is theestimate value of the measured distance, and u(L_(i-1)′) is themeasurement uncertainty of the estimate value L_(i-1)′ of the measureddistance when the synthetic wavelength is λ_(Si-1); when i=2,L_(i-1)′=L₁, namely L₁′=L₁;

For each synthetic wavelength λ_(Si), repeat Step 2). When thewavelength λ₂ of the second laser is changed, the moving displacement ofthe first cube-corner prism is recorded as Δl_(i) during eachmeasurement. The following approach is adopted to perform measurementduring each measurement:

According to the wavelength-leverage relationship, the coarsemeasurement value L_(i) of the measured absolute distance correspondingto the fractional part of the synthetic wavelength λ_(Si) is calculatedby the computer using the following equation:

$L_{i} = {{\frac{\lambda_{S\; i}}{\lambda_{1}} \cdot \Delta}\; l_{i}}$

With the coarse measurement value L_(i) of the measured absolutedistance, the fractional fringe order ε_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L is calculated by thecomputer using the following equation:

$ɛ_{Si} = \frac{2L_{i}}{\lambda_{Si}}$

Then, with the estimate value L_(i-1)′ of the measured absolute distanceand the fractional fringe order ε_(Si) of the synthetic wavelengthλ_(Si), the integral fringe order M_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance is calculated by thecomputer using the following equation:

$M_{Si} = {{int}\left\lbrack {\frac{2L_{i - 1}^{\prime}}{\lambda_{Si}} + 0.5 - ɛ_{Si}} \right\rbrack}$

where int[ ] is rounded down.

Step 4) specifically includes:

4.1) For the measuring process with respect to i=2, 3, . . . , n, eachestimate value L_(i)′ of the measured absolute distance L is calculatedby the computer using the following equation:

$L_{i}^{\prime} = {\left( {M_{Si} + ɛ_{Si}} \right) \cdot \frac{\lambda_{Si}}{2}}$

When 4u(L_(n)′)<λ₁ is satisfied, where u(L_(n)′) is the measurementuncertainty of the estimate value L_(n)′ of the measured absolutedistance when the final synthetic wavelength is λ_(Sn), the measuredabsolute distance L is calculated by the computer using the followingequation:

$L = {\left( {M_{n} + ɛ_{n}} \right) \cdot \frac{\lambda_{1}}{2}}$

where M_(n) represents the integral fringe order of the wavelength λ₁included in the measured absolute distance L, ε_(n) represents thefractional fringe order of the wavelength λ₁ included in the measuredabsolute distance L, and M_(n) and ε_(n) are respectively calculated byusing the following equations:

${M_{n} = {{int}\left\lbrack {\frac{2L_{n}^{\prime}}{\lambda_{1}} + 0.5 - ɛ_{n}} \right\rbrack}}\mspace{45mu}$$ɛ_{n} = \frac{2\; \Delta \; l_{n}}{\lambda_{1}}$

where L_(n)′ is the estimate value of the measured absolute distance Land Δl_(n) is the moving displacement of the first cube-corner prism inthe final measurement.

The wavelength λ₁ output from the first laser is a fixed wavelength andthe wavelength λ₂ output from the second laser can be changed.

Compared with the background of the invention, the present invention hasthe advantages that:

(1) Using the wavelength leverage principle, the measured distance oflarge length is converted into the measurement of easily detecteddisplacement of the cube-corner prism in the reference arm, instead ofacquiring the fractional fringe order of the synthetic wavelengththrough phase determination. The method is easy to realize;

(2) The method can realize the absolute measurement of arbitrarydistance. And when the synthetic wavelength is gradually decreased andtransited to the single wavelength, the measurement accuracy of themeasured distance can reach nanometer level. Thus the method has highmeasurement accuracy.

(3) The method can realize the transition from synthetic wavelength tosingle wavelength. The method is simple in structure, low in cost andconvenient to use.

The present invention is mainly applied to the measurement of absolutedistance of large length or large size with high accuracy in the fieldof large precision equipment manufacturing, spatial engineering andmetrological technique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the optical configuration of the present invention.

FIG. 2 is the schematic diagram of the absolute distance measurementusing wavelength leverage of the present invention.

In the figures: 10. first laser, 11. second laser, 12. first beamexpander, 13. second beam expander, 14. first reflector, 15. firstpolarizing beam splitter, 20. first beam splitter, 21. second polarizingbeam splitter, 22. first cube-corner prism, 23. second beam splitter,24. first shutter, 25. second shutter, 26. second reflector, 27. thirdreflector, 28. third beam splitter, 29. second cube-corner prism, 210.third cube-corner prism, 30. third polarizing beam splitter, 31. firstphotodetector, 32. second photodetector, 33. data acquisition module,34. computer, and 35. controller.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is further described in details hereinafter withthe Figures and Embodiments.

As shown in FIG. 1, the apparatus of the present invention comprises alight source system I, a wavelength-leverage laser interferometricsystem II and an interference signal processing and controlling systemIII. The light source system I outputs a orthogonally linearly polarizedbeam with the wavelength λ₁ and wavelength λ₂. The orthogonally linearlypolarized beam projects onto the wavelength-leverage laserinterferometric system II to form the interference beam. Theinterference beam projects onto the interference signal processing andcontrolling system III. The controller 35 in the interference signalprocessing and controlling system III is used for controlling the changeof the wavelength λ₂ in the light source system I.

The light source system I comprises a first laser 10, a second laser 11,a first beam expander 12, a second beam expander 13, a first reflector14 and a first polarizing beam splitter 15. The linearly polarized beamwith a constant wavelength λ₁ and the polarization direction parallel tothe page plane emitted from the first laser 10 passes through the firstbeam expander 12 and projects onto the first polarizing beam splitter15. The linearly polarized beam with a variable wavelength λ₂ and thepolarization direction perpendicular to the page plane emitted from thesecond laser 11 passes through the second beam expander 13, is reflectedby the first reflector 14 and projects onto the first polarizing beamsplitter 15. The polarization direction of the beam from the first laser10 is perpendicular to that of the beam from the second laser 11. InFIG. 1, the vertical short line in the light path represents the beamwith the wavelength λ₁ whose polarization direction is parallel to thepage plane, and the black dot represents the beam with the wavelength λ₂whose polarization direction is perpendicular to the page plane. Thelinearly polarized beam with the wavelength λ₁ transmitted by the firstpolarizing beam splitter 15 and the linearly polarized beam with thewavelength λ₂ reflected by the first polarizing beam splitter 15 combineone orthogonally linearly polarized beam.

The wavelength-leverage laser interferometric system II comprises afirst beam splitter 20, a second polarizing beam splitter 21, a firstcube-corner prism 22, a second beam splitter 23, a first shutter 24, asecond shutter 25, a second reflector 26, a third reflector 27, a thirdbeam splitter 28, a second cube-corner prism 29 and a third cube-cornerprism 210. Being incident on the first beam splitter 20, theorthogonally linearly polarized beam is divided into the reflectedreference beam and the transmitted measurement beam. The reference beamprojects onto the second polarizing beam splitter 21, wherein thelinearly polarized beam with the wavelength λ₂ is reflected twice by thesecond polarizing beam splitter 21 and projects onto the first beamsplitter 20, and the linearly polarized beam with the wavelength λ₂passes through the second polarizing beam splitter 21, projects onto thefirst cube-corner prism 22, returns after being reflected, and projectsonto the first beam splitter 20 after passing through the secondpolarizing beam splitter 21 again. Being incident on the second beamsplitter 23, the measurement beam is divided into the reflected near-endmeasurement beam and the transmitted far-end measurement beam. Thenear-end measurement beam projects onto the first beam splitter 20 afterpassing through the first shutter 24, the second reflector 26, thesecond cube-corner prism 29 and the third beam splitter 28 in sequence.And the far-end measurement beam projects onto the first beam splitter20 after passing through the second shutter 25, the third cube-cornerprism 210, the third reflector 27 and the third beam splitter 28 insequence. Being reflected by the first beam splitter 20, the near-end orfar-end measurement beam recombines with the reference beam transmittedby the first beam splitter 20 to form the interference beam.

The interference signal processing and controlling system III comprisesa third polarizing beam splitter 30, a first photodetector 31, a secondphotodetector 32, a data acquisition module 33, a computer 34 and acontroller 35. The interference beam from the wavelength-leverage laserinterferometric system II projects onto the third polarizing beamsplitter 30. The interference beam with the wavelength λ₂ reflected bythe third polarizing beam splitter 30 is received by the firstphotodetector 31, and the interference beam with the wavelength λ₁transmitted by the third polarizing beam splitter 30 is received by thesecond photodetector 32. The interference signals output by the twophotodetectors are respectively sent to the data acquisition module 33,and then transmitted to the computer 34 after being processed by thedata acquisition module 33. According to the calculated result, thecomputer 34 changes the value of the wavelength λ₂ emitted from thesecond laser 11 by means of the controller 35.

In the light source system I, the wavelength λ₁ of the beam emitted fromthe first laser 10 is a constant value and the wavelength λ₂ of the beamemitted from the second laser 11 is a variable value.

In the wavelength-leverage laser interferometric system II, thereference arm is composed of the second polarizing beam splitter 21 andthe first cube-corner prism 22 while the measurement arm is composed ofthe second beam splitter 23, the first shutter 24, the second shutter25, the second reflector 26, the third reflector 27, the third beamsplitter 28, the second cube-corner prism 29 and the third cube-cornerprism 210.

In the wavelength-leverage laser interferometric system II, there is aone-to-one correspondence between the synthetic wavelength λ_(S) formedby the linearly polarized beams with the wavelengths λ₁ and λ₂ in themeasurement arm and the wavelength λ₁ of the linearly polarized beamprojecting onto the first cube-corner prism 22 in the reference arm. Themeasured absolute distance in the measurement arm and the movingdisplacement of the first cube-corner prism 22 in the reference arm forma leverage relationship.

In the wavelength-leverage laser interferometric system II, when thefirst shutter 24 is opened and the second shutter 25 is closed, thenear-end measurement beam incident on the first beam splitter 20 and thereference beam incident on the first beam splitter 20 form interference.When the first shutter 24 is closed and the second shutter 25 is opened,the far-end measurement beam incident on the first beam splitter 20 andthe reference beam incident on the first beam splitter 20 forminterference. That is, the near-end measurement beam and the far-endmeasurement beam respectively interfere with the reference beam byalternately opening and closing the first shutter 24 and the secondshutter 25.

In the embodiment of the present invention, the first laser 10, which isone tunable diode laser with the model of DL Pro 633 made by TopticaCompany from Germany, emits the constant wavelength λ₁ of 631 nm. Thesecond laser 11, which is another tunable diode laser with the model ofDL Pro 633 made by Toptica Company from Germany, emits the wavelength λ₂with the range of 630 nm-637 nm. The first photodetector 31 and thesecond photodetector 32 are PIN photodetectors with the model of S09105made by Beijing Suoyang Photoelectric Technology Co., Ltd. The dataacquisition module 33 is a data acquisition card with the model ofPCI-9820 made by Adlink Technology Company. The computer 34 is a desktopcomputer with the model of Pro4500 made by the HP Company. And thecontroller 35 is a controller with the model of Digilock110 made byToptica Company from Germany.

Combined with FIG. 2, the specific implementation for measuring absolutedistance using laser interferometric wavelength leverage includes thefollowing steps:

1) The first laser 10 emits the linearly polarized beam with a constantwavelength λ₁. Controlling the second laser 11 to emit the linearlypolarized beam with a variable wavelength λ₂ makes one half of theprimary synthetic wavelength λ_(S1) formed by the wavelengths λ₁ and λ₂be larger than the measured absolute distance L, which is the distancebetween the second cube-corner prism 29 and the third cube-corner prism210;

2) Adjust the open and close states of the first shutter 24 and thesecond shutter 25. Move the first cube-corner prism 22 along the axialdirection of light path. Establish the wavelength-leverage relationship.The primary coarse measurement value of the measured absolute distanceis achieved;

2.1) When the first shutter 24 is opened and the second shutter 25 isclosed, the near-end measurement beam returned from the secondcube-corner prism 29 interferes with the reference beam. Theinterference signals of the wavelength λ₂ and wavelength λ₁ detectedrespectively by the first photodetector 31 and the second photodetector32 are respectively shown as follows:

$\begin{matrix}{\phi_{2} = {4\pi \frac{L_{R}}{\lambda_{2}}}} & (1) \\{\phi_{1} = {4{\pi\left( {\frac{L_{R}}{\lambda_{1}} - \frac{L_{M}}{\lambda_{1}}} \right)}}} & (2)\end{matrix}$

where φ₁ and φ₂ are the phases of the interference signals of thewavelengths λ₁ and λ₂ detected by the first photodetector 31 and thesecond photodetector 32, respectively. L_(R) is the distance differencebetween the first beam splitter 20 to the second cube-corner prism 29and the first beam splitter 20 to the second polarizing beam splitter21. And L_(M) is the distance between the second polarizing beamsplitter 21 and the first cube-corner prism 22. λ₁ and λ₂ are the laserwavelengths in the air, λ₁=λ₁₀/n₁ and λ₂=λ₂₀/n₂ (λ₁₀ and λ₂₂ are thelaser wavelengths in the vacuum, n₁ and n₂ are the refractive index ofair, n₁ and n₂ are calculated with the Edlen equation by measuring thetemperature, humidity, pressure and CO₂ concentration of the air);

Moving the first cube-corner prism 22 along the axial direction of lightpath makes the phase difference between the two interference signalsdetermined by the data acquisition module 33 be equal to zero, that is,there is:

$\begin{matrix}{{\Delta\phi} = {{\phi_{2} - \phi_{1}} = {{4{\pi\left( {\frac{L_{R}}{\lambda_{S\; 1}} + \frac{L_{M\; 1}}{\lambda_{1}}} \right)}} = 0}}} & (3)\end{matrix}$

where L_(M1) is the distance between the second polarizing beam splitter21 and the first cube-corner prism 22 with respect to the primarysynthetic wavelength; λ_(S1)=λ₁λ₂/|λ₁−λ₂| is the primary syntheticwavelength formed by λ₁ and λ₂;

2.2) When the first shutter 24 is closed and the second shutter 25 isopened, the far-end measurement beam returned from the third cube-cornerprism 210 interferes with the reference beam. As the measured distance Lis introduced, the phase difference Δφ′ between the two interferencesignals is changed as follows:

$\begin{matrix}{{\Delta\phi}^{\prime} = {4{\pi\left( {\frac{L_{R} + L}{\lambda_{S\; 1}} + \frac{L_{M\; 1}}{\lambda_{1}}} \right)}}} & (4)\end{matrix}$

In order to measure the measured distance L, the first cube-corner prism22 is moved a smaller distance Δl (less than λ₁/2) along the axialdirection of light path, namely L_(M1)→L_(M1)±Δl. The phase differenceΔφ″ between the two interference signals is changed as follows:

$\begin{matrix}{{\Delta\phi}^{''} = {4{\pi\left( {\frac{L_{R} + L}{\lambda_{S\; 1}} + \frac{L_{M\; 1} \pm {\Delta \; l}}{\lambda_{1}}} \right)}}} & (5)\end{matrix}$

When λ₂>/λ₁, the first cube-corner prism 22 moves towards the secondpolarizing beam splitter 21, and the sign before Δl is plus. Whenλ₂</λ₁, the first cube-corner prism 22 moves far away from the secondpolarizing beam splitter 21, and the sign before Δl is minus;

2.3) The smaller distance Δl moved by the first cube-corner prism 22shall satisfy to make Δφ″=0. According to Eq. (3) and Eq. (5), thewavelength-leverage relationship among the primary synthetic wavelengthλ_(S1) the wavelength λ₁, the measured absolute distance L and themoving displacement Δl of the first cube-corner prism 22 is establishedas follows:

$\begin{matrix}{\frac{L}{\lambda_{S\; 1}} + \frac{\Delta \; l}{\lambda_{1}}} & (6)\end{matrix}$

The wavelength-leverage relationship is shown in FIG. 2. The primarycoarse measurement value L₁ of the measured absolute distance iscalculated by the computer 34 as follows:

$\begin{matrix}{L_{1} = {{\frac{\lambda_{S\; 1}}{\lambda_{1}} \cdot \Delta}\; l}} & (7)\end{matrix}$

3) Keep the wavelength λ₁ output from the first laser 10 constant. Thecomputer 34 changes the wavelength λ₂ output from the second laser 11 bymeans of the controller 35 so that the wavelength λ₁ and the wavelengthλ₂ form a series of synthetic wavelengths λ_(S2), λ_(S3), . . . ,λ_(Si), . . . , λ_(Sn). The measurement is performed repeatedly withrespect to each change of the wavelength λ₂. The fractional fringe orderε_(Si) and the integral fringe order M_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L are obtained throughcalculation;

A series of synthetic wavelengths meets λ_(S2)>λ_(S3)> . . . >λ_(Sn),and each synthetic wavelength λ_(Si) meets:

λ_(Si)>4u(L _(i-1)′), i=2,3, . . . ,n

where i represents the sequence number of measurement times, nrepresents the total number of measurement times, L_(i-1)′ is theestimate value of the measured distance, and u(L_(i-1)′) is themeasurement uncertainty of the estimate value L_(i-1)′ of the measureddistance when the synthetic wavelength is λ_(Si-1) (when i=L_(i-1)′=L₁,that is L₁′=L₁);

For each synthetic wavelengths λ_(Si), repeat Steps 2.1) and 2.2). Whenthe wavelength λ₂ of the second laser 11 is changed every time, themoving displacement of the first cube-corner prism 22 is recorded asΔl_(i). According to Eq. (4), the phase difference Δφ′″ between the twointerference signals during each measurement is changed to:

$\begin{matrix}{{\Delta\phi}^{\prime\prime\prime} = {4{\pi\left( {\frac{L_{R} + L_{i}^{\prime}}{\lambda_{Si}} + \frac{L_{M\; i}}{\lambda_{1}}} \right)}}} & (8)\end{matrix}$

where L_(i)′ is the estimate value of the measured distance L, andL_(Mi) is the distance between the second polarizing beam splitter 21and the first cube-corner prism 22 with respect to the syntheticwavelength λ_(Si);

In Eq. (8), when the measured distance L is larger than λ_(Si)/2, Δφ′″includes the integral number and the fractional part of 2π. Δφ′″ isexpressed as another expression:

$\begin{matrix}{{\Delta\phi}^{\prime\prime\prime} = {{2{\pi \left( {M_{Si} + ɛ_{Si}} \right)}} + {4{\pi\left( {\frac{L_{R}}{\lambda_{Si}} + \frac{L_{M\; i}}{\lambda_{1}}} \right)}}}} & (9)\end{matrix}$

According to Eq. (8) and Eq. (9), the estimate value L_(i)′ of themeasured distance is expressed as:

$\begin{matrix}{L_{i}^{\prime} = {\left( {M_{Si} + ɛ_{Si}} \right) \cdot \frac{\lambda_{Si}}{2}}} & (10)\end{matrix}$

where M_(n) and ε_(n) respectively represent the integral fringe orderand the fractional fringe order of the synthetic wavelength λ_(Si)included in the measured distance;

Repeat Step 2.3). the coarse measurement value L_(i) of the measuredabsolute distance corresponding to the fractional part of the syntheticwavelength λ_(Si) is calculated by the computer 34 as follows:

$\begin{matrix}{L_{i} = {{\frac{\lambda_{Si}}{\lambda_{1}} \cdot \Delta}\; l_{i}}} & (11)\end{matrix}$

The fractional fringe order of the synthetic wavelength λ_(Si) includedin the measured absolute distance L is given by:

$\begin{matrix}{ɛ_{Si} = \frac{2L_{i}}{\lambda_{Si}}} & (12)\end{matrix}$

With the estimate value L_(i-1)′ of the measured absolute distancedetermined by last synthetic wavelength λ_(Si-1) and the fractionalfringe order ε_(Si) currently determined, the integral fringe orderM_(Si) of the synthetic wavelength λ_(Si) included in the measuredabsolute distance L is given by:

$\begin{matrix}{M_{Si} = {{int}\left\lbrack {\frac{2L_{i - 1}^{\prime}}{\lambda_{Si}} + 0.5 - ɛ_{Si}} \right\rbrack}} & (13)\end{matrix}$

where int[ ] is rounded down.

4) According to the synthetic wavelength λ_(Si) and its fractionalfringe order ε_(Si) and integral fringe order M_(Si) obtained in theabove steps, the computer 34 calculates each estimate value of themeasured absolute distance L during each measurement. The measuredabsolute distance L is obtained through calculation. Then themeasurement of absolute distance using laser interferometric wavelengthleverage is realized.

4.1) For i=2, 3, . . . , n, substituting the obtained M_(Si) and ε_(Si)into Eq. (10), the estimate value L_(i)′ of the measured absolutedistance corresponding to the synthetic wavelength λ_(Si) is calculatedby the computer 34;

Substituting Eq. (11) into Eq. (12), we get:

$\begin{matrix}{ɛ_{Si} = \frac{2\Delta \; l_{i}}{\lambda_{1}}} & (14)\end{matrix}$

Substituting Eq. (14) into Eq. (10), we get:

$\begin{matrix}{L_{i}^{\prime} = {{M_{Si} \cdot \frac{\lambda_{Si}}{2}} + {{\frac{\lambda_{Si}}{\lambda_{1}} \cdot \Delta}\; l_{i}}}} & (15)\end{matrix}$

From Eq. (15), the measurement uncertainty of the estimate value L_(i)′of the measured absolute distance is derived as follows:

$\begin{matrix}{{u\left( L_{i}^{\prime} \right)} = \sqrt{\begin{matrix}{\left( {\frac{M_{Si}}{2} \cdot {u\left( \lambda_{Si} \right)}} \right)^{2} + \left( {\frac{\lambda_{Si}}{\lambda_{1}} \cdot {u\left( {\Delta \; l_{i}} \right)}} \right)^{2} + \left( {\frac{\Delta \; l_{i}}{\lambda_{1}} \cdot {u\left( \lambda_{Si} \right)}} \right)^{2} +} \\\left( {\frac{\Delta \; l_{i}\lambda_{Si}}{\lambda_{1}^{2}} \cdot {u\left( \lambda_{1} \right)}} \right)^{2}\end{matrix}}} & (16)\end{matrix}$

Equation (16) shows that the measurement uncertainty u(L_(i)′) of theestimate value L_(i)′ of the measured absolute distance depends on theinteger value M_(Si) of the synthetic wavelength λ_(Si), the uncertaintyu(λ_(Si)) of the synthetic wavelength λ_(Si), the uncertainty u(λ₁) ofthe wavelength λ₁ and the uncertainty u(Δl_(i)) of the movingdisplacement Δl_(i) of the first cube-corner prism 22;

For different i, repeat Step 2). When the condition of 4u(L_(n)′)<λ₁ issatisfied, the measured absolute distance is calculated by the computer34 according to the following equation:

$\begin{matrix}{L = {\left( {M_{n} + ɛ_{n}} \right) \cdot \frac{\lambda_{1}}{2}}} & (17)\end{matrix}$

where M represents the integral fringe order of the wavelength λ₁included in the measured absolute distance L, ε_(n) represents thefractional fringe order of the wavelength λ₁ included in the measuredabsolute distance L, and M_(n) and ε_(n) are respectively calculated byusing the following equations:

$M_{n} = {{{{int}\left\lbrack {\frac{2L_{n}^{\prime}}{\lambda_{1}} + 0.5 - ɛ_{n}} \right\rbrack}\mspace{31mu} ɛ_{n}} = \frac{2\Delta \; l_{n}}{\lambda_{1}}}$

where L_(n)′ is the estimate value of the measured absolute distance Land Δl_(n) is the moving displacement of the first cube-corner prism 22in the final measurement.

For the case that the condition of 4u(L_(n)′)<λ₁ is not satisfied, Step3) is continuously repeated until the condition of 4u(L_(n)′)<λ₁ issatisfied, then the measured absolute distance L is calculated by usingEq. (17).

The absolute distance of 50 m is measured as an example. When thewavelength λ₁ output from the first laser 10 is equal to 631 m,u(λl_(i)) is equal to 0.0004 μm, the relative uncertainty of the vacuumwavelengths λ₁₀ and λ₂₀ is 10⁻¹⁰ and the relative uncertainty of themeasurement of the air refractive index is 10⁻⁹, the relativeuncertainty of the wavelengths λ₁, λ₂ and λ_(Si) in the air is 10⁻⁹,after measurements with four synthetic wavelengths are performed, themeasurement uncertainty of the measured absolute distance reaches 0.04μm, this satisfies the condition of 4u(L_(n)′)<λ₁. The specific data isshown in Table 1.

TABLE 1 Four synthetic wavelength values and corresponding measurementresults when the measured absolute distance L = 50 m Moving displacementEstimate value Relative Synthetic Δl_(i) of first L_(i)′ of theMeasurement measurement Wavelength wavelength cube-corner absoluteuncertainty accuracy of λ₂ λ_(Si) prism 22 distance u(L_(i)′) thedistance No. (nm) (μm) (μm) (μm) (μm) L − L_(i)′/L 1 631.0000020199861638.70 0.1576 49917898.98 95021.38 1.64 × 10⁻³ 2 631.00026561498962.29 0.2248 49999775.43 712.66 4.49 × 10⁻⁶ 3 631.0332048 11991.700.0321 49999996.18 5.70 7.63 × 10⁻⁸ 4 635.6829255 85.65 0.104849999999.96 0.06  8.76 × 10⁻¹⁰

According to Eq. (17), the measured absolute distance L=49999999.98 μmis obtained through calculation, and the relative accuracy of themeasurement of the absolute distance is 3.54×10⁻¹⁰.

From the above description, these show that the present invention hasrealized precision measurement of the absolute distance of large length.The present invention has the advantages of high measurement accuracy,simple optical configuration, convenient to use and remarkable technicaleffects.

The above embodiment is intended to explain the present invention, butnot to limit the present invention. Any modification and change made tothe present invention within the protection scope of the spirit and theClaims of the present invention fall in the protection scope of thepresent invention.

1. An absolute distance measurement apparatus using laserinterferometric wavelength leverage, comprising a light source system(I), a wavelength-leverage laser interferometric system (II) and aninterference signal processing and controlling system (III), wherein thelight source system (I) outputs a orthogonally linearly polarized beamwith the wavelength λ₁ and wavelength λ₂, the orthogonally linearlypolarized beam projects onto the wavelength-leverage laserinterferometric system (II) to form the interference beam, theinterference beam projects onto the interference signal processing andcontrolling system (III), and the controller in the interference signalprocessing and controlling system (III) is used for controlling thechange of the wavelength λ₂ in the light source system (I).
 2. Theabsolute distance measurement apparatus using laser interferometricwavelength leverage according to claim 1, wherein the light sourcesystem (I) comprises a first laser (10), a second laser (11), a firstbeam expander (12), a second beam expander (13), a first reflector (14)and a first polarizing beam splitter (15), wherein the linearlypolarized beam with a constant wavelength λ₁ emitted from the firstlaser (10) passes through the first beam expander (12) and projects ontothe first polarizing beam splitter (15), the linearly polarized beamwith a variable wavelength λ₂ emitted from the second laser (11), whosepolarization direction is perpendicular to that of the beam from thefirst laser (10), passes through the second beam expander (13), isreflected by the first reflector (14) and projects onto the firstpolarizing beam splitter (15), and the linearly polarized beam with theconstant wavelength λ₁ transmitted by the first polarizing beam splitter(15) and the linearly polarized beam with the variable wavelength λ₂reflected by the first polarizing beam splitter (15) combine oneorthogonally linearly polarized beam.
 3. The absolute distancemeasurement apparatus using laser interferometric wavelength leverageaccording to claim 1, wherein the wavelength-leverage laserinterferometric system (II) comprises a first beam splitter (20), asecond polarizing beam splitter (21), a first cube-corner prism (22), asecond beam splitter (23), a first shutter (24), a second shutter (25),a second reflector (26), a third reflector (27), a third beam splitter(28), a second cube-corner prism (29) and a third cube-corner prism(210), wherein being incident on the first beam splitter (20), theorthogonally linearly polarized beam is divided into the reflectedreference beam and the transmitted measurement beam, wherein thereflected reference beam projects onto the second polarizing beamsplitter (21), wherein the linearly polarized beam with the wavelengthλ₂ is reflected twice by the second polarizing beam splitter (21) andprojects onto the first beam splitter (20), and the linearly polarizedbeam with the wavelength λ₁ passes through the second polarizing beamsplitter (21), projects onto the first cube-corner prism (22), returnsafter being reflected, and projects onto the first beam splitter (20)after passing through the second polarizing beam splitter (21) again,wherein being incident on the second beam splitter (23), the transmittedmeasurement beam is divided into the reflected near-end measurement beamand the transmitted far-end measurement beam, wherein the near-endmeasurement beam projects onto the first beam splitter (20) afterpassing through the first shutter (24), the second reflector (26), thesecond cube-corner prism (29) and the third beam splitter (28) nsequence, and the far-end measurement beam projects onto the first beamsplitter (20) after passing through the second shutter (25), the thirdcube-corner prism (210), the third reflector (27) and the third beamsplitter (28) in sequence, wherein being reflected by the first beamsplitter (20), the near-end or far-end measurement beam recombines withthe reference beam transmitted by the first beam splitter (20) to formthe interference beam.
 4. The absolute distance measurement apparatususing laser interferometric wavelength leverage according to claim 1,wherein the interference signal processing and controlling system (III)comprises a third polarizing beam splitter (30), a first photodetector(31), a second photodetector (32), a data acquisition module (33), acomputer (34) and a controller (35), wherein the interference beam fromthe wavelength-leverage laser interferometric system (II) projects ontothe third polarizing beam splitter (30), wherein the interference beamwith the wavelength λ₂ reflected by the third polarizing beam splitter(30) is received by the first photodetector (31), and the interferencebeam with the wavelength λ₁ transmitted by the third polarizing beamsplitter (30) is received by the second photodetector (32), wherein theinterference signals output by the two photodetectors are respectivelysent to the data acquisition module (33), and then transmitted to thecomputer (34) after being processed by the data acquisition module (33),wherein according to the calculated result, the computer (34) changesthe value of the wavelength λ₂ emitted from the second laser (11) bymeans of the controller (35).
 5. The absolute distance measurementapparatus using laser interferometric wavelength leverage according toclaim 2, wherein in the light source system (I), the wavelength λ₁ ofthe laser beam emitted from the first laser (10) is a constant value andthe wavelength λ₂ of the laser beam emitted from the second laser (11)is a variable value.
 6. The absolute distance measurement apparatususing laser interferometric wavelength leverage according to claim 3,wherein in the wavelength-leverage laser interferometric system (II),the reference arm is composed of the second polarizing beam splitter(21) and the first cube-corner prism (22) while the measurement arm iscomposed of the second beam splitter (23), the first shutter (24), thesecond shutter (25), the second reflector (26), the third reflector(27), the third beam splitter (28), the second cube-corner prism (29)and the third cube-corner prism (210).
 7. The absolute distancemeasurement apparatus using laser interferometric wavelength leverageaccording to claim 3, wherein in the wavelength-leverage laserinterferometric system (II), there is a one-to-one correspondencebetween the synthetic wavelength λ_(S) formed by the linearly polarizedbeams with the wavelengths λ₁ and λ₂ in the measurement arm and thewavelength λ₁ of the linearly polarized beam projecting onto the firstcube-corner prism (22) in the reference arm, wherein the measuredabsolute distance in the measurement arm and the moving displacement ofthe first cube-corner prism (22) in the reference arm form a leveragerelationship.
 8. An absolute distance measurement method using laserinterferometric wavelength leverage applied to the apparatus accordingto claim 1, wherein the method adopts the apparatus to performmeasurement, and the specific steps are as follows: 1) The first laser(10) emits the linearly polarized beam with the wavelength, whereincontrolling the second laser (11) to emit the linearly polarized beamwith the wavelength λ₂ makes one half of the primary syntheticwavelength λ_(S1) formed by the wavelengths λ₁ and λ₂ be larger than themeasured absolute distance L which is the distance between the secondcube-corner prism (29) and the third cube-corner prism (210); 2) Adjustthe open and close states of the first shutter (24) and the secondshutter (25), move the first cube-corner prism (22) along the axialdirection of light path, and establish the wavelength-leveragerelationship, wherein the primary coarse measurement value of themeasured absolute distance is achieved; 3) Keep the wavelength λ₁ outputfrom the first laser (10) constant, wherein the computer (34) changesthe wavelength λ₂ output from the second laser (11) by means of thecontroller (35) so that the wavelength λ₁ and the wavelength λ₂ form aseries of synthetic wavelengths λ_(S2), λ_(S3), . . . , λ_(Si), . . . ,λ_(Sn), wherein the measurement is performed repeatedly with respect toeach change of the wavelength λ₂, wherein the fractional fringe orderε_(Si) and the integral fringe order M_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L are obtained throughcalculation; 4) According to the synthetic wavelength λ_(Si) and itsfractional fringe order ε_(Si) and integral fringe order M_(Si) obtainedin the above steps, the computer (34) calculates each estimate value ofthe measured absolute distance L during each measurement, wherein themeasured absolute distance L is obtained through calculation, and thenthe measurement of absolute distance using laser interferometricwavelength leverage is realized.
 9. The absolute distance measurementmethod using laser interferometric wavelength leverage applied to theapparatus according to claim 8, wherein Step 2) specifically includes:2.1) When the first shutter (24) is opened and the second shutter (25)is closed, the near-end measurement beam returned from the secondcube-corner prism (29) and the reference beam form the interferencesignals, and moving the first cube-corner prism (22) along the axialdirection of light path makes the phase difference Δφ between theinterference signals of wavelengths λ₂ and λ₁ detected respectively bythe first photodetector (31) and the second photodetector (32) be equalto zero; 2.2) When the first shutter (24) is closed and the secondshutter (25) is opened, the far-end measurement beam returned from thethird cube-corner prism (210) and the reference beam form theinterference signals, wherein the phase difference Δφ between the twointerference signals received by the first photodetector (31) and thesecond photodetector (32) is changed. Moving again the first cube-cornerprism (22) along the axial direction of light path makes the phasedifference Δφ be equal to zero, wherein the moving displacement of thefirst cube-corner prism (22) is recorded as Δl; 2.3) Thewavelength-leverage relationship among the primary synthetic wavelengthλ_(S1), the wavelength λ₁ output from the first laser (10), the measuredabsolute distance L and the moving displacement Δl of the firstcube-corner prism (22) is established as follows:$\frac{L}{\lambda_{S\; 1}} = \frac{\Delta \; l}{\lambda_{1}}$ whereλ_(S1)=λ₁λ₂/|λ₁−λ₂| is the primary synthetic wavelength formed by thewavelengths λ₁ and λ₂, wherein λ₁ and λ₂ are the wavelengths output fromthe first laser (10) and the second laser (11), respectively; Accordingto the wavelength-leverage relationship, the primary coarse measurementvalue L₁ of the measured absolute distance is calculated by the computer(34) using the following equation:$L_{1} = {{\frac{\lambda_{S\; 1}}{\lambda_{1}} \cdot \Delta}\; {l.}}$10. The absolute distance measurement method using laser interferometricwavelength leverage applied to the apparatus according to claim 8,wherein Step 3) specifically includes: A series of synthetic wavelengthsmeets λ_(S2)>λ_(S3)> . . . >λ_(Sn), and each synthetic wavelength λ_(Si)meetsλ_(Si)>4u(L _(i-1)′), i=2,3, . . . ,n where i represents the sequencenumber of measurement times, n represents the total number ofmeasurement times, L_(i-1)′ is the estimate value of the measureddistance, and u(L_(i-1)′) is the measurement uncertainty of the estimatevalue L_(i-1)′ of the measured distance when the synthetic wavelength isλ_(Si-1); when i=2, L_(i-1)′=L₁, namely L₁′=L₁; For each syntheticwavelength λ_(Si), repeat Step 2), when the wavelength λ₂ of the secondlaser (11) is changed, the moving displacement of the first cube-cornerprism (22) is recorded as Δl_(i) during each measurement, wherein thefollowing approach is adopted to perform measurement during eachmeasurement: According to the wavelength-leverage relationship, thecoarse measurement value L_(i) of the measured absolute distancecorresponding to the fractional part of the synthetic wavelength λ_(Si)is calculated by the computer (34) using the following equation:$L_{i} = {{\frac{\lambda_{S\; i}}{\lambda_{1}} \cdot \Delta}\; l_{i}}$With the coarse measurement value L_(i) of the measured absolutedistance, the fractional fringe order ε_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L is calculated by thecomputer (34) using the following equation:$ɛ_{Si} = {\frac{2L_{i}}{\lambda_{Si}}.}$ Then, with the estimate valueL_(i-1)′ of the measured absolute distance and the fractional fringeorder ε_(Si) of the synthetic wavelength λ_(Si), the integral fringeorder M_(Si) of the synthetic wavelength λ_(Si) included in the measuredabsolute distance is calculated by the computer (34) using the followingequation:$M_{Si} = {{int}\left\lbrack {\frac{2L_{i - 1}^{\prime}}{\lambda_{Si}} + 0.5 - ɛ_{Si}} \right\rbrack}$where int[ ] is rounded down.
 11. The absolute distance measurementmethod using laser interferometric wavelength leverage applied to theapparatus according to claim 8, wherein Step 4) specifically includes:4.1) For the measuring process with respect to i=2, 3, . . . , n, eachestimate value L_(i)′ of the measured absolute distance L is calculatedby the computer (34) using the following equation:$L_{i}^{\prime} = {\left( {M_{Si} + ɛ_{Si}} \right) \cdot \frac{\lambda_{Si}}{2}}$When 4u(L_(n)′)<λ₁ is satisfied, where u (L_(n)′) is the measurementuncertainty of the estimate value L_(n)′ of the measured absolutedistance when the final synthetic wavelength is λ_(Sn), the measuredabsolute distance L is calculated by the computer (34) using thefollowing equation:$L = {\left( {M_{n} + ɛ_{n}} \right) \cdot \frac{\lambda_{1}}{2}}$ whereM_(n) represents the integral fringe order of the wavelength λ₁ includedin the measured absolute distance L, ε_(n) represents the fractionalfringe order of the wavelength λ₁ included in the measured absolutedistance L, and M_(n) and ε_(n) are respectively calculated by using thefollowing equations:$M_{n} = {{{{int}\left\lbrack {\frac{2L_{n}^{\prime}}{\lambda_{1}} + 0.5 - ɛ_{n}} \right\rbrack}\mspace{31mu} ɛ_{n}} = \frac{2\Delta \; l_{n}}{\lambda_{1}}}$where L_(n)′ is the estimate value of the measured absolute distance Land Δl_(n) is the moving displacement of the first cube-corner prism(22) in the final measurement.
 12. The absolute distance measurementmethod using laser interferometric wavelength leverage applied to theapparatus according to claim 8, wherein the wavelength λ₁ output fromthe first laser (10) is a fixed wavelength and the wavelength λ₂ outputfrom the second laser (11) can be changed.
 13. An absolute distancemeasurement method using laser interferometric wavelength leverageapplied to the apparatus according to claim 2, wherein the method adoptsthe apparatus to perform measurement, and the specific steps are asfollows: 1) The first laser (10) emits the linearly polarized beam withthe wavelength, wherein controlling the second laser (11) to emit thelinearly polarized beam with the wavelength λ₂ makes one half of theprimary synthetic wavelength λ_(S1) formed by the wavelengths λ₁ and λ₂be larger than the measured absolute distance L which is the distancebetween the second cube-corner prism (29) and the third cube-cornerprism (210); 2) Adjust the open and close states of the first shutter(24) and the second shutter (25), move the first cube-corner prism (22)along the axial direction of light path, and establish thewavelength-leverage relationship, wherein the primary coarse measurementvalue of the measured absolute distance is achieved; 3) Keep thewavelength λ₁ output from the first laser (10) constant, wherein thecomputer (34) changes the wavelength λ₂ output from the second laser(11) by means of the controller (35) so that the wavelength λ₁ and thewavelength λ₂ form a series of synthetic wavelengths λ_(S2), λ_(S3), . .. , λ_(Si), . . . , λ_(Sn), wherein the measurement is performedrepeatedly with respect to each change of the wavelength λ₂, wherein thefractional fringe order ε_(Si) and the integral fringe order M_(Si) ofthe synthetic wavelength λ_(Si) included in the measured absolutedistance L are obtained through calculation; 4) According to thesynthetic wavelength λ_(Si) and its fractional fringe order ε_(Si) andintegral fringe order M_(Si) obtained in the above steps, the computer(34) calculates each estimate value of the measured absolute distance Lduring each measurement, wherein the measured absolute distance L isobtained through calculation, and then the measurement of absolutedistance using laser interferometric wavelength leverage is realized.14. An absolute distance measurement method using laser interferometricwavelength leverage applied to the apparatus according to claim 3,wherein the method adopts the apparatus to perform measurement, and thespecific steps are as follows: 1) The first laser (10) emits thelinearly polarized beam with the wavelength, wherein controlling thesecond laser (11) to emit the linearly polarized beam with thewavelength λ₂ makes one half of the primary synthetic wavelength λ_(S1)formed by the wavelengths λ₁ and λ₂ be larger than the measured absolutedistance L which is the distance between the second cube-corner prism(29) and the third cube-corner prism (210); 2) Adjust the open and closestates of the first shutter (24) and the second shutter (25), move thefirst cube-corner prism (22) along the axial direction of light path,and establish the wavelength-leverage relationship, wherein the primarycoarse measurement value of the measured absolute distance is achieved;3) Keep the wavelength λ₁ output from the first laser (10) constant,wherein the computer (34) changes the wavelength λ₂ output from thesecond laser (11) by means of the controller (35) so that the wavelengthλ₁ and the wavelength λ₂ form a series of synthetic wavelengths λ_(S2),λ_(S3), . . . , λ_(Si), . . . , λ_(Sn), wherein the measurement isperformed repeatedly with respect to each change of the wavelength λ₂,wherein the fractional fringe order ε_(Si) and the integral fringe orderM_(Si) of the synthetic wavelength λ_(Si) included in the measuredabsolute distance L are obtained through calculation; 4) According tothe synthetic wavelength λ_(Si) and its fractional fringe order ε_(Si)and integral fringe order M_(Si) obtained in the above steps, thecomputer (34) calculates each estimate value of the measured absolutedistance L during each measurement, wherein the measured absolutedistance L is obtained through calculation, and then the measurement ofabsolute distance using laser interferometric wavelength leverage isrealized.
 15. An absolute distance measurement method using laserinterferometric wavelength leverage applied to the apparatus accordingto claim 4, wherein the method adopts the apparatus to performmeasurement, and the specific steps are as follows: 1) The first laser(10) emits the linearly polarized beam with the wavelength, whereincontrolling the second laser (11) to emit the linearly polarized beamwith the wavelength λ₂ makes one half of the primary syntheticwavelength λ_(S1) formed by the wavelengths λ₁ and λ₂ be larger than themeasured absolute distance L which is the distance between the secondcube-corner prism (29) and the third cube-corner prism (210); 2) Adjustthe open and close states of the first shutter (24) and the secondshutter (25), move the first cube-corner prism (22) along the axialdirection of light path, and establish the wavelength-leveragerelationship, wherein the primary coarse measurement value of themeasured absolute distance is achieved; 3) Keep the wavelength λ₁ outputfrom the first laser (10) constant, wherein the computer (34) changesthe wavelength λ₂ output from the second laser (11) by means of thecontroller (35) so that the wavelength λ₁ and the wavelength λ₂ form aseries of synthetic wavelengths λ_(S2), λ_(S3), . . . , λ_(Si), . . . ,λ_(Sn), wherein the measurement is performed repeatedly with respect toeach change of the wavelength λ₂, wherein the fractional fringe orderε_(Si) and the integral fringe order M_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L are obtained throughcalculation; 4) According to the synthetic wavelength λ_(Si) and itsfractional fringe order ε_(Si) and integral fringe order M_(Si) obtainedin the above steps, the computer (34) calculates each estimate value ofthe measured absolute distance L during each measurement, wherein themeasured absolute distance L is obtained through calculation, and thenthe measurement of absolute distance using laser interferometricwavelength leverage is realized.
 16. An absolute distance measurementmethod using laser interferometric wavelength leverage applied to theapparatus according to claim 5, wherein the method adopts the apparatusto perform measurement, and the specific steps are as follows: 1) Thefirst laser (10) emits the linearly polarized beam with the wavelength,wherein controlling the second laser (11) to emit the linearly polarizedbeam with the wavelength λ₂ makes one half of the primary syntheticwavelength λ_(S1) formed by the wavelengths λ₁ and λ₂ be larger than themeasured absolute distance L which is the distance between the secondcube-corner prism (29) and the third cube-corner prism (210); 2) Adjustthe open and close states of the first shutter (24) and the secondshutter (25), move the first cube-corner prism (22) along the axialdirection of light path, and establish the wavelength-leveragerelationship, wherein the primary coarse measurement value of themeasured absolute distance is achieved; 3) Keep the wavelength λ₁ outputfrom the first laser (10) constant, wherein the computer (34) changesthe wavelength λ₂ output from the second laser (11) by means of thecontroller (35) so that the wavelength λ₁ and the wavelength λ₂ form aseries of synthetic wavelengths λ_(S2), λ_(S3), . . . , λ_(Si), . . . ,λ_(Sn), wherein the measurement is performed repeatedly with respect toeach change of the wavelength λ₂, wherein the fractional fringe orderε_(Si) and the integral fringe order M_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L are obtained throughcalculation; 4) According to the synthetic wavelength λ_(S1) and itsfractional fringe order ε_(Si) and integral fringe order M_(Si) obtainedin the above steps, the computer (34) calculates each estimate value ofthe measured absolute distance L during each measurement, wherein themeasured absolute distance L is obtained through calculation, and thenthe measurement of absolute distance using laser interferometricwavelength leverage is realized.
 17. An absolute distance measurementmethod using laser interferometric wavelength leverage applied to theapparatus according to claim 6, wherein the method adopts the apparatusto perform measurement, and the specific steps are as follows: 1) Thefirst laser (10) emits the linearly polarized beam with the wavelength,wherein controlling the second laser (11) to emit the linearly polarizedbeam with the wavelength λ₂ makes one half of the primary syntheticwavelength λ_(S1) formed by the wavelengths λ₁ and λ₂ be larger than themeasured absolute distance L which is the distance between the secondcube-corner prism (29) and the third cube-corner prism (210); 2) Adjustthe open and close states of the first shutter (24) and the secondshutter (25), move the first cube-corner prism (22) along the axialdirection of light path, and establish the wavelength-leveragerelationship, wherein the primary coarse measurement value of themeasured absolute distance is achieved; 3) Keep the wavelength λ₁ outputfrom the first laser (10) constant, wherein the computer (34) changesthe wavelength λ₂ output from the second laser (11) by means of thecontroller (35) so that the wavelength λ₁ and the wavelength λ₂ form aseries of synthetic wavelengths λ_(S2), λ_(S3), . . . , λ_(Si), . . . ,λ_(Sn), wherein the measurement is performed repeatedly with respect toeach change of the wavelength λ₂, wherein the fractional fringe orderε_(Si) and the integral fringe order M_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L are obtained throughcalculation; 4) According to the synthetic wavelength λ_(Si) and itsfractional fringe order ε_(Si) and integral fringe order M_(Si) obtainedin the above steps, the computer (34) calculates each estimate value ofthe measured absolute distance L during each measurement, wherein themeasured absolute distance L is obtained through calculation, and thenthe measurement of absolute distance using laser interferometricwavelength leverage is realized.
 18. An absolute distance measurementmethod using laser interferometric wavelength leverage applied to theapparatus according to claim 7, wherein the method adopts the apparatusto perform measurement, and the specific steps are as follows: 1) Thefirst laser (10) emits the linearly polarized beam with the wavelength,wherein controlling the second laser (11) to emit the linearly polarizedbeam with the wavelength λ₂ makes one half of the primary syntheticwavelength λ_(S1) formed by the wavelengths λ₁ and λ₂ be larger than themeasured absolute distance L which is the distance between the secondcube-corner prism (29) and the third cube-corner prism (210); 2) Adjustthe open and close states of the first shutter (24) and the secondshutter (25), move the first cube-corner prism (22) along the axialdirection of light path, and establish the wavelength-leveragerelationship, wherein the primary coarse measurement value of themeasured absolute distance is achieved; 3) Keep the wavelength λ₁ outputfrom the first laser (10) constant, wherein the computer (34) changesthe wavelength λ₂ output from the second laser (11) by means of thecontroller (35) so that the wavelength λ₁ and the wavelength λ₂ form aseries of synthetic wavelengths λ_(S2), λ_(S3), . . . , λ_(Si), . . . ,λ_(Sn), wherein the measurement is performed repeatedly with respect toeach change of the wavelength λ₂, wherein the fractional fringe orderε_(Si) and the integral fringe order M_(Si) of the synthetic wavelengthλ_(Si) included in the measured absolute distance L are obtained throughcalculation; 4) According to the synthetic wavelength λ_(Si) and itsfractional fringe order ε_(Si) and integral fringe order M_(Si) obtainedin the above steps, the computer (34) calculates each estimate value ofthe measured absolute distance L during each measurement, wherein themeasured absolute distance L is obtained through calculation, and thenthe measurement of absolute distance using laser interferometricwavelength leverage is realized.